Attention swimmers: More than half of the public pools tested in a new study contained bacterial evidence that someone may have pooped in the pool.

Investigators from the Centers for Disease Control and Prevention worked with state and local public health departments last summer to collect samples from pool filters at 161 pools in the metro-Atlanta area. Some of the pools were public, some were in private clubs and some were in water parks.

Over the winter, researchers used genetic tests to identify several types of pathogens in the filter samples. Among the 161 samples, 93 -- or 58% -- contained Escherichia coli, a bacterium that lives in the digestive tract of humans and other warm-blooded animals. The researchers treated the presence of E. coli as “a fecal indicator,” they wrote in their report.

How did it get into the pools? In all likelihood, swimmers delivered some of it into the water by failing to take a thorough, soapy shower before getting into the pool. “Each person has an average of 0.14 grams of fecal material on their perianal surface that could rinse into the water,” the report notes.

Larger quantities of E. coli could be introduced through “a formed or diarrheal fecal incident in the water.” Diarrhea is particularly troubling because it’s more likely to transmit pathogens to other swimmers, according to the study.

Pools in private clubs were less likely to have E. coli in their filters, but investigators still found it in 49% of cases. Municipal pools had the highest incidence of E. coli -- 70% -- followed by water parks, at 66%.

In one bright spot, none of the filters had evidence of the strain of E. coli known as O157:H7, which causes serious -- sometimes deadly -- cases of foodborne illness.

While E. coli was the most disgusting find, it wasn’t the most common. Investigators found Pseudomonas aeruginosa in 95 of the 161 filter samples, for a prevalence of 59%. The P. aeruginosa bacterium can cause swimmer’s ear, an inflammation of the outer ear and ear canal. It also causes itchy skin.

P. aeroginosa is a “ubiquitous microbe” that can make its way into pools via people, dirt, kickboard and other foam items, or even the water used to fill the pool in the first place. Once in the pool, it forms biofilms on pool walls, plumbing equipment and other submerged surfaces. This pathogen can be kept in check with proper pool maintenance, including use of chlorine and disinfection with ultraviolet light, according to the report.

Of the 161 pool filters tested, 121 (or 75%) were found to have at least one of the microbes investigators tested for. In addition, 67 (42%) of the pools had both E. coli and P. aeroginosa. Other pathogens were much less common – Giardia intestinalis was discovered on two filters, and Cryptosporidium spp. was found once. Other no-shows besides E. coli O157:H7 were norovirus GI, norovirus GII and adenovirus.

The results were published Thursday in the CDC’s Morbidity and Mortality Weekly Report.

The researchers emphasized that the results of their Atlanta-area survey can't be generalized to the rest of the United States. But they noted that cases of recreational water illness have been on the rise from coast to coast, suggesting "that swimmers frequently introduce fecal material and pathogens into recreational water throughout the country." Reassuringly, there were no pool-related health outbreaks in Georgia during the months when the filter samples were collected.

Though poor maintenance was surely a factor in these pools, the study authors said that swimmers shoulder some of the blame.

“Swimmers have the power and responsibility to decrease the risk for RWIs by practicing good hygiene,” they wrote. That includes staying out of the water when sick with diarrhea; showering off with soap before entering the pool; taking bathroom breaks every 60 minutes; and rinsing off again before getting back into the pool. People caring for small children should be sure to check diapers every 30 to 60 minutes and refrain from changing diapers poolside to prevent pathogens from rinsing into the water.

Fecal matter isn’t the only body residue that can cause trouble in a pool. Urine and sweat contain nitrogen, which combines with free chlorine in the water to form compounds called di- and tri-chloramines that irritate the eyes and respiratory tract, the study says.